The long range goal of this research is to develop a mechanical ventilator that has the potential to reduce morbidity and mortality in critically ill pediatric patients. Our novel approach controls the pressure in the patient's airway using a unique jet nozzle and valve system. The conventional approach uses a valve placed at the end of several feet of exhalation tubing. Our new nozzle will jet gas down the patient's endotracheal tube for inspiration and control of expiratory pressure. The potential advantages of this innovative design are that one ventilator can be used: 1) for patients ranging in size from the very low birth weight preterm infant to the 30 kg child; 2) in either the pressure-limited time-cycled mode or a volume control mode; 3) at both conventional rates and high frequency rates (ranging from 1 to 660 breaths/min); 4) to reduce CO2 in the lung's anatomic dead space so that smaller tidal volumes may be used; 5) to assist the patient's own breathing effort, even in the tiny preterm infant, because of its rapid response time capabilities. Completion of the research plan will result in a valve-nozzle system which will simplify and improve gas delivery to infants and children who are dependent on mechanical ventilation. It will also reduce risks related to pressure and resistance to spontaneous breathing that are known to accompany conventional ventilation.